The present invention relates to a toner for developing an electrostatic latent image formed on an image carrier by an electrophotographic process or an ion-flow method, and manufacturing methods thereof.
Image forming apparatuses, which employ an electrophotographic method, create an image by fixing a toner image on a recording medium. Examples of the image forming apparatuses are a laser printer, a Light Emitting Diode (LED) printer and a digital photocopying machine.
In the electrophotographic method, an electrostatic latent image, which is in accordance with image information, is visualized by the toner (a developer) thereby creating a visible image. The visible image (a toner image) is transcribed and fixed on the recording medium. The electrostatic latent image is produced by electrifying the entire surface of a photoreceptor, then by irradiating the surface with light in accordance with the image information by use of a laser beam or LED. The visible image is created by the visualization of the electrostatic latent image with the toner (the developer) by a developing section. The fixation of the visible image onto the recording medium is carried out at a fixation section by fixing on the recording medium the visible image of the toner which was transferred onto the recording medium at a transfer section.
There has been greater demand for more compact image forming apparatuses, recently. In an image forming apparatus of the electrophotographic method, a toner storing section is targeted for size reduction to achieve miniaturization of the image forming apparatus because its occupying space is significantly large in the image forming apparatus. A large quantity of the toner must be stored in the image forming apparatus for user""s convenience since the image forming apparatus may be used by more than one person and with a great number of printout, especially in the recent network environment.
Demand for color image output also has been increased recently. A color image forming apparatus, in which toners of three or four colors are used, needs a much larger space for a toner storing section in the image forming apparatus. Moreover, a bulky fixation section is necessary in case of a color image because the color is expressed by multi-color overlapping with a greater consumption of the toner on the recording medium such as paper or an Over Head Projection (OHP) sheet, thus requiring a greater application of heat for the thermal fixation, compared to the case of a monochrome image.
In addition, there is demand for a further energy-saving and environment-friendly method for manufacturing the toner. Today""s common manufacturing methods of the toner are: (a) a method that involves melting, kneading, and grinding processes (MKG method), which has been employed conventionally, and (b) polymerization methods in a liquid solvent, which has been introduced recently. Known as the polymerization methods are, for example, suspension polymerization, emulsion polymerization, and dispersion polymerization methods.
Generally, the quantity of a coloring agent (carbon black or color pigments) contained in the toner is ranging from a few % up to about 10% by weight. The quantity of the toner necessary for achieving necessary image-intensity is between about 0.7 mg/cm2 and 1 mg/cm2. Due to those requirements, the image forming apparatus should store a large quantity of the toner, as described above.
Therefore, the quantity of the toner necessary for expressing the image information can be reduced by increasing the coloring agent content in the toner, thereby resulting in a smaller space occupied by the toner storing section in the image forming apparatus.
However, poor dispersibility of the coloring agent in the toner is often caused in the conventional toner and the conventional methods thereof when the coloring agent content in the toner is increased for reducing the quantity of toner used. With the poor dispersibility, the increase in the coloring agent content reduces the coloring power of the coloring agent, on the contrary.
Moreover, it is very difficult to further improve the dispersibility of the coloring agent in the polymerization methods. For example, the suspension polymerization method, which is the most popular among the polymerization methods, has difficulty in increasing the coloring agent content further than the current level with satisfactory dispersibility maintained. It is because the re-agglomeration of coloring agent particles tends to occur during the polymerization reaction with the increase in the quantity of the coloring agent, besides the problem in uniform dispersion of the raw materials (a mixture of monomers or coloring agents).
In addition, with respect to the dispersion of the coloring agent, the MKG method has an advantage over the polymerization methods that the large shear force is large in the melting and the kneading processes and the re-agglomeration of the coloring agent particles is prevented by a rapid cooling process following the kneading process.
However, for the toner produced by the MKG method, the way of manufacturing the toner, that is, preparing a chip of a resin by melting and kneading, then grinding down the chip to targeted particle diameters, leads to susceptibility of the chip to cleavage at its resin part containing the coloring agent particles during the grinding process, so that the toner has a structure with a number of the coloring agent particles exposed from the surface of the toner. This has an adverse effect on the electric characteristics (charge characteristics) of the toner.
Moreover, the toner of the MKG method is mechanically weak at the interface between the coloring agent particles and the binding resin when a large quantity of the coloring agent particles is mixed in, so that stabile production of the toner with targeted particle-size distribution cannot be achieved due to the damage on the particles during the grinding process.
Further considering effects on the environment, the polymerization methods require some environmental measures such as washing and waste-fluid treatments for their organic solvents used in a large quantity. Furthermore, because the toner is produced in a liquid, the polymerization methods need a drying process that consumes a huge quantity of energy.
In view of the foregoing conventional problems, the present invention has an object to provide a toner and its manufacturing methods, by which a desirable image quality is achieved with a small quantity of the toner, as well as energy saving, by use of a supercritical fluid (a SCF) or a sub-supercritical fluid (a sub-SCF) while increasing a coloring agent content in the toner and maintaining dispersibility of the coloring agent.
In order to solve the above problems, a toner manufacturing method of the present invention includes at least the steps of (a) dissolving a binding resin component in a SCF or a sub-SCF so that the binding resin component is blended with a coloring agent component, (b) lowering solubility of the binding resin component so that the binding resin component is precipitated in particle shapes, thus producing a toner with the coloring agent component dispersed in an interior of the binding resin component precipitated in the particle shapes.
With the above method, use of the SCF or the sub-SCF gives good dispersibility of the coloring agent component in the thus produced toner, even when the coloring agent content is increased. This maintains good image formation free from the conventional problems, such as low coloring power due to the increase in the coloring agent content, and unstable toner charge characteristics resulted from the exposure of the coloring agent component.
Moreover, the above method can promote miniaturization of the image forming apparatus using the toner by reducing the quantity of the toner used by way of increasing the content of the coloring agent. Furthermore, shorter precipitation time for the binding resin component can reduce the energy and production cost of the toner production with the above method, compared to the conventional polymerization methods and the MKG method for manufacturing a toner.
In order to solve the foregoing problems, another toner manufacturing method of the present invention includes the steps of (a) blending core-forming toner particles, which include at least the binding resin component and coloring agent component, with a surface modifier component to be applied on the surface of the core-forming toner particles in a SCF or a sub-SCF, (b) lowering solubility of the surface modifier component after dissolving the surface modifier component so that the surface modifier component is precipitated on the surface of the core-forming toner particles, thereby producing a surface modified toner.
Therefore, with the above method, the exposure of the coloring agent component from the surface of the toner can be reduced by coating the surface with the surface modifier component. Thus, excellent charge characteristics of the product toner are achieved even for the toner with high coloring power given by containing a large quantity of the coloring agent component in the core-forming toner particles.
Furthermore, the toner prepared by the above method also has excellent mechanical strength by being coated with the surface modifier component. This can reduce breakdown of the toner after a long usage, thereby providing stable images as well as lowering the energy consumption and production cost of the toner manufacturing.
The toner of the present invention is manufactured by either of the manufacturing methods, in order to solve the foregoing conventional problems. For the solution of the forgoing conventional problems, another toner of the present invention is composed of a binding resin component of particle shapes and coloring agent component dispersed in the binding resin component, in which the dispersion of the coloring agent component is carried out using the SCF or the sub-SCF.
In the above arrangement, accordingly, the coloring power of the coloring agent component can be maintained due to excellent dispersibility of the coloring agent component, even when a coloring agent content is set to be large.
As a result, the above arrangement contributes to the miniaturization of the image forming apparatus using the toner, still maintaining the excellent image forming ability even with less toner.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.